The invention relates generally to MR imaging and, more particularly, to a method and apparatus of split-blade data collection for PROPELLER MRI in combination with a quadratic phase modulation scheme.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In certain clinical imaging applications, it is desirable to acquire “diffusion-weighted” images in which tissues that have either higher or lower water self-diffusion characteristics relative to other tissues are emphasized. Typically, diffusion-weighting is implemented using a pair of large gradient pulses bracketing a refocusing RF pulse. Because spins undergoing irregular motion due to diffusion are not completely re-phased by the second gradient pulse of the pair, signal from these spins is attenuated such that tissues with higher water diffusion experience increased signal loss.
Most clinical diffusion-weighted imaging is performed using single-shot sequences, such as single-shot echo-planar imaging (EPI). However, single-shot acquisitions typically have limited resolution and are sensitive to susceptibility-induced image distortions and eddy-current effects. For multi-shot acquisitions, non-diffusive bulk motions can cause shot-specific phase shifts that can destructively interfere when the multiple shots are combined, resulting in image artifacts. To reduce image artifacts, these phase shifts may be corrected for each shot individually before combining shots into a final image. Multiple approaches for performing such a motion correction for multi-shot acquisitions are known in the art.
An alternative to diffusion-weighted EPI imaging is known as a diffusion-weighted fast spin-echo (FSE) method of imaging. Compared to diffusion-weighted EPI imaging, diffusion-weighted FSE methods for imaging are far less susceptible to B0-rated artifacts and can be implemented with less intensive gradient requirements. However, despite these advantages, diffusion-weighted FSE imaging has several challenges. First, there is an extremely high phase sensitivity to motion in diffusion-weighted FSE imaging which, if left uncorrected, can lead to motion artifacts. Secondly, in diffusion-weighted FSE, the signal will generally violate the Carr-Purcell -Meiboom-Gill (CPMG) condition. A CPMG sequence is a type of spin-echo pulse sequence consisting of a 90° radio frequency (RF) pulse followed by an echo train induced by successive 180° pulses. To meet the CPMG condition, the initial transverse magnetization must be aligned with the axis of the refocusing pulses. As the signal in diffusion-weighted FSE will generally not meet the CPMG condition, the signal is substantially degraded.
In an effort to address the issue of signal degradation in conventional diffusion-weighted FSE imaging, LeRoux proposed a quadratic phase modulation scheme in U.S. Pat. No. 6,265,873. The quadratic phase modulation scheme proposed by LeRoux, which will be referred to hereafter as the “LRX” phase modulation scheme, allowed the use of a low flip angle and a long echo train in FSE imaging. Unlike conventional diffusion-weighted FSE imaging, the LRX phase modulation scheme has been shown to sustain signal magnitude and phase, regardless of the signal phase at the beginning of the echo train. In LRX phase modulation, the odd and even echoes must be acquired at the same k-space location (i.e., double encoded) due to the difference between the odd and even echoes in the echo train. Two intermediate images are generated from the respective odd and even echoes, and the two intermediate images are then combined together to form a final image. Unfortunately, due to this double encoding, the echo train in an LRX modulation scheme must be twice as long as the echo train of a conventional FSE method to fill the same k-space matrix size. This, along with the combination of the two intermediate images, leads to an undesirably elongated acquisition time.
Yet another diffusion-weighted FSE imaging technique that has been developed is commonly referred to as diffusion-weighted Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) imaging. PROPELLER imaging is an FSE technique wherein an MR signal is encoded by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. In subsequent echo trains, this blade is incrementally rotated in k-space about the origin, thereby allowing adequate measurement for sufficient regions of k-space for a desired resolution. Diffusion-weighted PROPELLER imaging is particularly advantageous in that it has increased insensitivity to motion-induced phase errors. However, conventional diffusion-weighted PROPELLER imaging uses a phase modulation scheme known as “XY2” phase modulation. In the XY2 phase modulation scheme, the flip angle of the refocusing RF pulse is usually close to 180 degrees, which may cause a high level of specific absorption rate (SAR), as well as sensitivity to dielectric effect at 3 T. Additionally, the echo train in the XY2 phase modulation scheme is limited in length, since the signal decays quickly.
It would therefore be desirable to have a system and method of MR imaging implementing a PROPELLER or similar imaging protocol in combination with an LRX phase modulation scheme to reduce the scan time, dielectric artifacts, and/or background streaking artifacts that may be present in conventional diffusion-weighted FSE imaging.